PMEA and (S)-9-3-hydroxy-2-(phosphonomethoxy)propyl!adenine (S)-HPMPA! have been reported to exhibit potent and selective activity against a broad spectrum of viruses,.sup.5,6 including herpes simplex virus (types 1 and 2), varicella zoster virus, cytomegalovirus, hepatitis B virus,.sup.7 as well as human immunodeficiency virus (HIV)..sup.8-10 These compounds have been prepared by classical coupling of adenine,.sup.11-13 or its precursors such as 6-chloropurine, with an appropriate phosphonate side chain (2) in the presence of a base such as NaH, K.sub.2 CO.sub.3, or Cs.sub.2 CO.sub.3 at elevated temperatures, followed by hydrolysis. However, coupling reactions under these basic conditions often yield both N.sup.7 - and N.sup.9 -substituted derivatives;.sup.11-13 the adenine salt generated during the reaction course also attacks the phosphonate ethyl ester to form N.sup.9 -ethyladenine..sup.13 Accordingly, there is a need for an efficient method for preparing PMEA and analogues thereof with less side product contamination and under mild conditions.